Check engine light on, now stalling

Hi, I hope you can help me out. Mr car has been a nightmare since I bought it (second hand few months ago). The check engine light came on and the care turned off few time on low speed. Took it to the mechanic to scan for codes and it gave lean fuel. So far I have cleaned MAF and throttle body (then I did a re-learn for the throttle body as that was stuffed up when the mechanic cleaned it). The light was off for a day or two after the mechanic turned it off then came back on, the car was jolting while driving on the motorway. Rpm would drop from 3000 rpm plus to 1000 then keeps going, now it drops all the way to 0 and the car stalls when I am doing sixty to seventy mph, and it seems to struggle to turn back on, today it took three times for it to start in the middle of the motorway. When I parked at a petrol station and turned it off, after about ten minutes when I tried to turn it back on it turned on and back off, then turned it on and was okay. I have also changed the front and middle exhaust as it had two holes in it (mechanic said it was overloading the lambda). It seems as though when the check engine light is off there are no issues with the car. I called the mechanic today and he said he is not sure but could be the ECU. I was hoping someone here would be able to help me out as I use my car for work and I travel one hundred and twenty miles every day. Thanks!